Not applicable.
The present invention relates to compressed gas systems and, more particularly, to a system for controlling and operating a plurality of compressors to efficiently supply compressed gas to a compressed gas distribution system.
Compressed gas is widely used in industry. For example, almost every industrial facility from a small machine shop to a large pulp and paper mill has some type of compressed air system. In many cases, compressed air is considered to be a xe2x80x9cfourth utilityxe2x80x9d and is so vital that the facility cannot operate without it. Compressed air systems vary in size from a single five horsepower compressor to systems comprising many compressors and aggregating more than 50,000 hp. While compressed air is clean, readily available, and simple to use, a typical compressed air system operates with around 10% overall efficiency and compressed air is probably the most expensive form of energy in use in a facility. Improving system maintenance, reducing system pressure, and using more efficient compressors are recognized paths to improved efficiency for compressed gas systems.
For example, leakage can consume 20-30% of the compressor output. However, leakage can be reduced to less than 10% of compressor output by using good maintenance practices and reducing system pressure because leakage is proportional to the pressure drop across the leak. However, reducing the system pressure requires compressed air storage and an accurate control system to prevent the system pressure from dropping below an acceptable minimum level during periods of high or changing demand.
Referring to FIG. 1, a typical compressed gas system 20 comprises, generally, a supply side 22 (indicated by a dashed enclosure) and a demand side 24 (indicated by a dashed enclosure). The supply side 22 of the system typically includes one or more compressors 26, 28, 30, and 32, supply side piping 40 and air treatment equipment, such as an air dryer 38. Gas compressors are commonly constructed as a compressor package including a drive motor, compressor controls, and a number of accessories. The compressors 26, 28, 30, and 32 are each driven by a drive motor 34 and equipped with inlet filtration 48 to protect the compressors and downstream equipment from airborne particles in the inlet air, and a separator 50 to remove oil and moisture from the compressed gas leaving the compressor. Although not illustrated, gas compressor packages also commonly include intercoolers to cool the air between compression stages and aftercoolers to cool the compressed air discharged by the compressor. Typically, the compressor""s controls (not illustrated) respond to a pressure signal, obtained from pressure sensors 54, at the discharge of the compressor package to the supply piping 40.
The demand side 24 of the system comprises distribution piping 43, including a demand header 55, leading to a plurality of end use equipment and processes 42.
The supply 22 and demand 24 sides of the system are typically separated by an air receiver or accumulator 44 and, often, a flow control 46. The accumulator 44 provides storage for a volume of compressed air and reduces compressor sequencing by permitting compressors to continue to operate when demand is momentarily reduced and providing a source of compressed air when demand momentarily increases. An appropriately sized accumulator can be used to protect end use equipment and processes having critical pressure requirements by controlling the amount and rate of system pressure change in response to demand events. The flow control 46 reduces pressure fluctuations in the demand side by adjusting the flow of compressed gas from the accumulator 44 to the demand side 24 of the system in response to changes in demand by the end uses 42. However, the supply side 22 of a compressed gas system has, essentially, a fixed volume and the pressure in the supply side will decrease in response to consumption of gas by the demand side 24 unless and until there is sufficient flow from the compressors to replace the gas being transferred to the demand side of the system. Likewise, when demand is reduced, the supply pressure, typically measured in the supply side header 36 or the accumulator 44, will rise until the compressor output is reduced to match the demand. If the system does not include a flow control 46, changes in demand by the end uses 42 produce immediate changes in the pressure in the demand header 55 resulting in rapidly fluctuating pressure in the accumulator 44.
Compressed gas systems are designed to operate within a fixed pressure range and to deliver to end uses 42 a volume of gas that varies with end use demand at a pressure in excess of a minimum pressure. Referring to FIG. 2, to compensate for pressure drops in the distribution piping 43 and to ensure that end uses 42 receive gas at a pressure in excess of the minimum, a minimum demand pressure 60 is maintained in the demand header 55. Likewise, to compensate for pressure drops between the accumulator 44 and the demand header 55, a minimum supply pressure 62, in excess of the demand pressure 60, is maintained in the accumulator 44 or supply header 36. The compressor control system typically operates in a fixed pressure range 64 (indicated by a bracket) adding or removing compressor capacity from the system in response to pressure changes resulting from demand in an attempt to keep the supply pressure 66 above the minimum supply pressure 62.
Air compressors operate most efficiently at full load and are, therefore, typically switched in and out of the system when the local pressure, typically measured at the outlet of the compressor package (sensor 54) reaches the limits of a control range established for the compressor. The compressor may be sequenced by controlling the drive motor 34 with a simple start-stop pressure switch but frequent cycling can cause overheating and wear of the motor and compressor. To reduce cycling of the drive motor, many compressors include a load-unload control that enables disconnecting the output of the compressor from the system while the drive motor continues to drive the compressor at full speed. However, an unloaded compressor typically consumes 15-60% of full load power while delivering no useful work to the system and lengthy periods of unloaded operation are detrimental to system efficiency.
A modulating or throttling inlet control can be used on a rotary screw type compressor to adjust the compressor""s output so that it more closely matches demand keeping the system pressure within the control range without unloading the compressor. However, when fully throttled, these compressors continue to operate against the system""s pressure and typically consume approximately 70% of full power. A variable speed drive is a more energy efficient means of regulating the output of a compressor to more closely match the system demand, but variable speed drives are relatively expensive. In a typical compressor operating scenario, a plurality of compressors, each operating at full capacity, are sequenced in and out of the distribution system in an attempt to match the combined compressor output to the system demand. In the alternative, one or more compressors with variable output may be used to fine tune the total compressor output over some limited range to reduce sequencing of fixed capacity compressors.
Automatic compressor control systems comprise generally sequencing controls and network controls. Sequencing controls or sequencers are devices used to sequentially load and unload a plurality of compressors in response to changes in the local system pressure. Sequencers can provide a fairly tight control range for the compressor and can be arranged to alter the order in which compressors are sequenced to balance the duty cycle on a plurality of compressors. However, sequencers rely on a local pressure signal from the outlet of the compressor package and variations in pressure throughout a system resulting from pressure drops and dynamic pressure fluctuations limit the use sequencers to controlling compressors at a common location.
Network controls typically utilize microprocessor-based controllers to provide a combination of system control functions and control of individual compressors. The controllers are linked so that operating information at various points in the system and the status of compressors distributed about the facility can be shared. One of the linked controllers is typically designated as the leader providing system operating decisions in response to pressure at various points in the supply side of the system. The effect is a tight control range for individual compressors and a coordinated response to changes in supply side pressure. The initial cost of network controls can be high compared to sequencers but the cost is often offset by reductions in operating costs.
The efficiency of a gas compression system can be increased by reducing the pressure of the gas in the system. A two psi. reduction in system pressure can reduce the operating cost of a typical compressed air system by approximately 1%, reducing leakage and energy consumed to compress the gas to a higher pressure. However, even with very accurate controls for individual compressors, the range of operating pressures 64, and, therefore, the average supply pressure 70, is determined, in large part, by the sequential arrangement of the operating pressure ranges for the sequenced compressors. For example, compressor A is to be loaded when the supply pressure 66 in the supply header 36 or accumulator 44 drops below the lower limit 74 of its operating range 76 (indicated by a bracket) (adjusted for a pressure drop between the outlet of the compressor 54 and the supply header 36). If the volume of air being added to the system by compressor A exceeds the demand, the supply pressure 66 will rise but to avoid frequent cycling of the compressor, compressor A will not be unloaded until the supply pressure reaches the upper limit 68 of its operating range 76. If, on the other hand, demand again increases, the supply pressure 66 will drop until the lower limit 78 of the operating range 80 (indicated by a bracket) for compressor B is reached, causing compressor B to be sequenced. If the demand continues to increase, the system pressure may drop below the lower limit 82 of compressor C""s operating range 84 causing its capacity to be sequenced into the system. The lower limit 86 of the operating range 88 of the last compressor in the sequence (compressor D) is typically set somewhat above the minimum supply pressure 62 so that a rapid increase in demand will not cause the supply pressure 66 to drop below the minimum pressure before compressor D""s capacity can be added to the system. As a result, the operating supply pressure range 64 of the system is the sum of the overlapping individual operating ranges 76, 80, 84, and 88 of the system""s compressors and a pressure margin 90 that protects end uses from exposure to pressures below the minimum allowable supply pressure 62. Improving system efficiency by reducing the average supply pressure 70 is limited by the width of the operating pressure ranges for the individual compressors and the cumulative nature of the sequential pressure ranges for a plurality of compressors supplying compressed gas to the distribution system.
What is desired, therefore, is a system for controlling the operation of at least one compressor of a compressed gas distribution system that facilitates operation of the distribution system in a manner that reduces average pressure for improved system efficiency.